Silica gel may be described as a coherent, rigid three-dimensional polymeric network of contiguous particles of colloidal silica. Silica gel is formed in a liquid medium, usually water, alcohol, or a mixture thereof. The terms "silica aquagel" and "silica alcogel" describe silica gels in which the pores are filled with liquid, the predominant component(s) being water or alcohol(s) as the case may be. Silica aquagel is also known as "silica hydrogel. "
A "silica xerogel" is a silica gel from which the liquid medium has been removed and replaced by a gas, the structure being compressed and the porosity reduced significantly by the surface tension forces as the liquid is removed. As soon as liquid begins to evaporate from a gel at temperatures below the critical temperature, surface tension creates concave menisci in the gel's pores. As evaporation continues, the menisci retreat into the gel body, compressive forces build up around its perimeter, and the perimeter contracts drawing the gel body inward. Eventually surface tension causes significant collapse of the gel body and a reduction of volume, often as much as two-thirds or more of the original volume. This shrinkage has an important consequential effect, namely, it causes a significant reduction in the porosity, often as much as 90 to 95 percent depending on the system and pore sizes.
A "silica aerogel" is a silica gel from which the liquid has been removed in such a way as to prevent significant collapse or change in the structure as liquid is removed. Specifically, this has been done by heating the liquid-filled gel, usually a silica alcogel, in an autoclave while maintaining the prevailing pressure above the vapor pressure of the liquid until the critical temperature of the liquid has been exceeded, and then gradually releasing the vapor, usually by gradually reducing the pressure either incrementally or continuously, while maintaining the temperature above the critical temperature. The critical temperature is the temperature above which it is impossible to liquify a gas no matter how great a pressure is applied. At temperatures above the critical temperature, the distinction between liquid and gas phases disappears and so does their interface. In the absence of an interface between liquid and gas phases, there is no surface tension and hence menisci do not form. There are accordingly no surface tension forces to collapse the gel to a small volume compared with the original gel. Such a process may be termed "supercritical drying. "
Silica aerogels produced by supercritical drying have high porosities, on the order of from 50 to 99 percent by volume. The remaining few percent is a continuous silica skeleton.
A major problem with the supercritical drying process is that autoclaves capable of withstanding considerable pressures are required. For example, the critical pressure (i.e., the pressure at the critical temperature) of methanol is about 8.0 MPa while that of ethanol is about 6.4 MPa. Large autoclaves are exceedingly expensive, so most silica aerogel articles and/or batches tend to be small in size.